Understanding Classless Subnetting Concept

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CCNA Routing and Switching
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Understanding IPv4 Addressing System
Understanding Class A Network IP Addresses
Understanding Class B Network IP Addresses
Understanding Class C Network IP Addresses
Understanding Classful Network Mask
Understanding Classless Subnet Mask
Understanding Classless Subnetting Concept

Let’s dig into Understanding Classless Subnetting Concept. In my previous article Understanding Classless Subnet Mask, I have explained briefly why in the 90’s a new approach to distribute the IP Space Classless Subnet Masks was required to replace Classful Masks. In this article, I am going to dig deep into this approach for Subnetting (dividing) the Classful Networks into smaller Subnets.

Table of Contents

Subnetting Class A Networks

As mentioned, Classful A Networks use 255.0.0.0 Netmask, but it can uses Classful B Netmask 255.255.0.0, or Classful C default Netmask 255.255.255.0, however, it’s important to remember that default Classful Masking Rule is by Byte base, yet the Classless Subnetting Rule is by Bit base. Let me show you what I mean.

Abbreviation Symbols

n = Network
S = Subnet
s = Sub-Subnet
h = Hosts

Besides:

When “Subnet Bit” words are used: I am referring to 1
When “Host Bit” words are used: I am referring to 0

The following Subnet Masking technique based on moving the Network place value bit by bit at a time. You will notice how I started with the first subnet mask then little by little start moving bit at a time, up to the last possible Subnet Mask.

Analyzing 255.128.0.0 Subnet Mask

Subnet Mask:       255.128.0.0
Prefix:            /9 bits
Binary Equivalent: 11111111.10000000.00000000.00000000
Meaning:           nnnnnnnn.Shhhhhh.hhhhhhhh.hhhhhhhh
Increment Number:  128

Focusing on the 2nd byte 1000000, draw the Binary chart and plug the 2nd byte starting from the right.

128   64   32   16   8   4   2   1
  1    0    0    0   0   0   0   0

Once the first bit at the second byte turned ON, the byte get divided by 2, meaning, 256 / 2 = 128. Another way of saying: because the 1st bit on the 2nd byte represents the main origin of the Network, then you have created 2 Subnets.

The following draw shows how the 255.0.0.0 had all the hosts bits under One Network, and how later on the 255.128.0.0 divided the Hosts bits to 2 Subnets.

As you can see from the above image, the 1st bit on the second Byte from the left was turned ON which have divided the Classful Network to 2 subnets.

Laying out 255.128.0.0 or Prefix /9 Result

The following layout is for Private Network 10.0.0.0 with prefix/9. It shows how prefix /9 divided the Network to 2 Subnet IDs with 2 Broadcast Addresses.

Subnet ID Zero:    10.0.0.0
Broadcast Address: 10.127.255.255
Usable IP Range:   10.0.0.1 up to 10.127.255.254

Subnet ID 128:     10.128.0.0
Broadcast Address: 10.255.255.255
Usable IP Range:   10.128.0.1 up to 10.255.255.254

Remember, dividing the Network to Subnets, creates Multiple Networks (called Subnet IDs), and more than one Broadcast Address.

Using prefix /9 canceled the origin Network ID 10.0.0.0/8, and created 2 subnets:

Subnet ID Zero: 10.0.0.0/9 with broadcast 10.127.255.255; and,
Subnet ID 128: 10.128.0.0/9 with broadcast of 10.255.255.255.

And since Cisco Recommends No more than 510 IP Addresses per Subnet, prefix /9 is not practical to use neither in Public nor in private. It provides more than 510 usable IP addresses, and it would be impossible to be used by ISP for Public use, since it lease massive number of IP addresses which more than the Business needs or any Router needs.

You need smaller blocks of IP range such 510, 254, 128, 64 and 32, these blocks of IP range are mostly used by private Networks inside the company, ISP on the other hand, which is the Public side of our Internet, should release smaller Blocks such 16, 8, 4, and 2 for most businesses.

TIP: Network Address Translation (NAT) for home and business use, played big role in saving the IPv4 Space, it can share one Public IP Address among hundreds and thousands of computers. Besides, business can use Variable Length Subnet Masks (VLSM) as well before using the Public Block of IP Addresses if needed. VLSM simply means Sub-Subnetting.

Subnetting Network 10.0.0.0

The following IP address range of Subnet ID Zero when using 255.128.0.0 Subnet Mask a.k.a Prefix /9

First Subnet ID Zero: 10.0.0.0/9

First IP Address: 10.0.0.2
Subnet Mask:      255.128.0.0
Subnet ID:        10.0.0.0
Broadcast:        10.127.255.255
Gateway:          10.0.0.1
Last IP Address:  10.127.255.254

The following IP address range of Subnet ID 128 when using 255.128.0.0 Subnet Mask a.k.a Prefix /9

Second Subnet ID 128: 10.128.0.0/9

First IP Address: 10.128.0.2
Subnet Mask:      255.128.0.0
Subnet ID:        10.128.0.0
Broadcast:        10.255.255.255
Gateway:          10.128.0.1
Last IP Address:  10.255.255.254

I will continue Subnetting Network 10.0.0.0 but without laying out the usable IP Address range of each subnet due to space reason, however, the importance of the following examples is to show you what’s happening at the Binary level.

Subnetting 10.0.0.0 using Prefix /10 or 255.192.0.0

Using 255.192.0.0 prefix /10 will divide Network 10.0.0.0 for 4 Subnets.

Subnet Mask:       255.192.0.0
Prefix:            /10 Bits
Binary Equivalent: 11111111.11000000.00000000.00000000
Meaning:           nnnnnnnn.SShhhhhh.hhhhhhhh.hhhhhhhh
Increment Number:  64